The present invention relates generally to an improved surge arrester assembly having a sealed back-up air gap. More specifically, the invention relates to a novel contact for use with an over-voltage arrester, and an arrangement for sealing the contact with a canister, which also receives the arrester.
Given the current and voltage sensitivity, and the high cost of most modern electric and electronic equipment, users of that equipment often employ line surge protectors connected electrically in-line with the equipment. These line protectors prohibit the flow of current in the presence of a voltage surge having a magnitude greater than the tolerance of the equipment. To this end, many of these line protectors contain voltage arrester assemblies means in the form of a surge voltage or over-voltage arrester, which shunts the line to ground in the presence of a voltage greater than that tolerable by the equipment. The construction and operation of protectors employing surge voltage arresters is well known in the relevant art, as is evident, for example, from the patent of Gilberts, U.S. Pat. No. 4,241,374, assigned to the assignee of the present invention. The disclosure of that patent is incorporated herein by this reference.
Over-voltage arresters usually comprise a pair of spaced electrodes defining therebetween a spark gap which may be surrounded by air (as in a so-called carbon arrester) or by an appropriate, usually inert, gas (as in a so-called gas tube arrester). If an over-voltage (i.e. voltage exceeding the equipment tolerances) is present in the line to the equipment, that over-voltage will cause an arc across the spark gap continue to ground. This prevents the over-voltage condition damaging the equipment. The above-referenced Gilberts patent utilizes a gas tube arrester.
To prevent an over-current condition from damaging the equipment, a solder pellet is usually included in the protector, as part of the arrester assembly. When an over-current is present, the heat generated thereby melts the pellet. With the pellet melted, the arrangement is such that an electrical connection is made between the line and ground, again preventing damage to the equipment. One such arrangement is shown and described in the above-referenced Gilberts patent.
As shown in the referenced Gilberts patent, a secondary or back-up spark gap is defined between one electrode of the gas tube and a canister housing the arrester. To seal this back-up spark gap, a potted 0-ring is used. The O-ring is placed between the canister and an electrical contact, which forms the connection to ground, and a quantity of a potting compound is added above the O-ring.
While this type of seal works well in some employments, there is room for further improvements. The sealed back-up air gap construction of the present invention is intended to provide such an improvement.